In thermal spray deposition, material in powder, wire, or rod form is heated to near its melting point or just above and the molten or nearly molten particles accelerated in a gas stream to a high velocity before impacting on the surface to be coated, the substrate. On impact the particles flow into thin laminar splats and rapidly freeze and cool. The coating is made up of many layers of splats. Metallic, ceramic, cermet, and some polymeric materials may be deposited using thermal spray devices. A variety of thermal spray devices would include plasma, detonation gun, high velocity oxy-fuel, wire arc, and flame spray. The coatings are usually produced by moving the thermal spray device relative to the part being coated to distribute the material uniformly over the surface in multiple passes. This helps to control the temperature of the surface being coated and the residual stress in the coating.
In plasma spray deposition a gas, usually based on argon, nitrogen, hydrogen, and/or helium, is ionized by an electric arc as it flows through a nozzle in a plasma spray torch forming a high temperature, high velocity partially ionized plasma effluent. Coating material, usually in powder form, is injected into the plasma effluent and heated to near or just above its melting point and the molten or nearly molten particles accelerated in a gas stream to a high velocity before impacting on the surface to be coated, the substrate. On impact the particles flow into thin laminar splats and rapidly freeze and cool. The coating is made up of many layers of splats. Metallic, ceramic, cermet, and some polymeric materials may be deposited. The coatings are usually produced by moving the plasma spray device relative to the part being coated to distribute the material uniformly over the surface in multiple passes. This helps to control the temperature of the surface being coated and the residual stress in the coating.
The most important parameters that determine the microstructure and properties of the coatings include the temperature of the particles, their velocity, the extent to which they have reacted with the environment during deposition, the rate of deposition, the angle of impact, and the temperature of the substrate and previously deposited coating. The particles are heated (with the exception of the wire arc process) and accelerated by the gaseous effluent of the thermal spray device thus the temperature and velocity achieved are a function, in part, of the dwell time in the effluent. For plasma spray, the particles are heated and accelerated by the gaseous effluent and thus the temperature and velocity achieved are a function, in part, the dwell time in the effluent. The dwell time, in turn, is defined by the velocity of the particles and the distance (called the standoff) between the exit of the thermal spray device and the substrate. The temperature and velocity of the effluent of the thermal spray device decrease fairly rapidly on exit from the device. Therefore, there is an optimum standoff that allows sufficient distance or time for the particles to be heated and accelerated, but not so great that the effluent and particle temperatures and velocities begin to decline significantly. The angle of impact can have a major influence on the microstructure and properties of the coating. Generally, the optimum angle is at 90 degrees or normal to the substrate. As the angle becomes lower, the microstructure becomes more turbulent and less dense. The rate at which this degradation occurs is a function, in part, of the velocity and temperature of the particles on impact. The effective standoff and the sensitivity to the angle of deposition are particularly important when thermal spraying components with a complex shape. Thermal spray and plasma spray are inherently a line of sight process, and the size of the spray device and shape of the part being coated may limit how close the spray device can be brought to the part and still maintain an allowable angle of deposition. Thus it may not be possible to bring the device close enough to the surface to deposit the particles at a sufficient temperature, velocity, and angle of impact to produce a coating with a suitable microstructure.
The residual stress in coatings is a further property that must be controlled for optimum results. In most cases low residual stress is desired, but in one case the in-plane tensile stress is controllably managed to produce long vertical through-thickness segmentation cracks (Taylor, U.S. Pat. No. 5,073,433). Residual stress arises from a combination of factors. One is the shrinkage of the coating material being deposited as it cools from the solidification temperature. Other factors can include a peening-like mechanism as particles impact the coating and temperature cycling of the coating and substrate as the thermal spray device moves over the part in its traverse. Another source of coating stress is related to the rate and extent of heat absorption and dissipation during coating. These are controlled by various means including the level of pre-heat given the substrate before coating commences and the amount of auxiliary cooling used during coating. Accomplishing and controlling preheating can be very difficult, particularly when coating large or complex shapes. Yet another major source of coating stress is the thickness of the layer put down as the thermal spray device traverses over the substrate. The factors controlling this layer thickness are many, and include the spray rate (grams powder sprayed per unit time), the standoff, and the sweep rate of the torch over the substrate. If the spray rate is held high for higher production rates, and the standoff is essentially fixed due to the issues discussed above, most of the burden of controlling the stress in the coating falls upon the manipulation of the relative sweep rate of the torch over the substrate, also known as the surface speed. To reduce stress, the surface speed is increased, and could approach high speeds such as 10,000 inches per minute. It may be possible to obtain these speeds for coating simple shapes, but not for complex shapes such as turbine blades that require robotic machines that can only achieve surface speeds on the order of 1000 inches per minute to optimize standoff and angle of deposition during coating. If the standoff could be increased and the desired coating properties still maintained, the surface speed could be reduced commensurately.
The effluent of a thermal spray device, i.e., plasma spray torch begins to mix with the surrounding environmental gases, usually air, immediately upon exiting the thermal spray device. If a reactive material is being deposited, such as most metals, polymeric materials, and, to a lesser extent, carbides and nitrides, the oxygen from the air being mixed with the effluent can oxidize the material, significantly changing the properties of the coating. Generally, the longer the standoff, the greater the degree of oxidation. There are two major methods of avoiding this oxidation. One is to deposit the coating in a vacuum chamber under a low pressure of inert gas. In this situation the inert gas, usually argon, is drawn into the effluent rather than air, and no oxidation occurs. This technique has been well developed for plasma spray deposition and can be very effective. It has an additional benefit of a longer standoff due to the low pressure environment. The capital and operating costs of such a system are very high, however, and the production rate is low. The alternative is to provide a coaxial inert gas shield or shroud surrounding the effluent. In this manner inert gas is drawn into the effluent of the thermal spray device, and oxidation of the coating material is prevented.
An effective gas shield is that invented by Jackson, U.S. Pat. No. 3,470,347. This invention provides a uniform flow of turbulent inert gas, usually argon, surrounding the effluent of a plasma spray torch. It is very effective in preventing oxidation of reactive materials during deposition, but it has a limited standoff capability. Thus, when coating parts with a complex shape such as one with deep fillets or protuberances, it may not be possible to keep the thermal spray device close enough to the surface to maintain effective shielding.
Another invention provides a laminar gas shield by introducing a flow of inert gas normal to the thermal spray effluent within the thermal spray nozzle or an attachment to the thermal spray device through a porous medium arrayed parallel to the effluent such that the interaction of the inert gas to the flow of the effluent will prevent infiltration of gas and/or vapor from the surrounding environment (M. S. Nowotarski, et al, U.S. Pat. No. 5,486,383).
One the many important fields of application for thermal spray coatings is that of thermal barriers on many of the components of gas turbine engines. Modern gas turbine engines for aircraft propulsion and for ground-based electrical power generation continue to push to higher operating temperatures, because overall efficiency improves with higher temperature Some gas turbines operate at such high temperatures that the directly heated metallic components, such as combustors, blades and vanes would have very short life if not given a protective ceramic coating. The ceramic coating, known as a thermal barrier coating (TBC), is an insulator and acts to reduce the substrate temperature.
There are many variations of thermal barrier coatings, based on the materials selected for the coating and the coating processes. Most TBCs include a metallic bondcoat applied to the metallic substrate component and, on top of the bondcoat, a ceramic layer, usually based on zirconium oxide because of its very low thermal conductivity. The zirconia layer of the coating varies depending on the specific requirements; e.g., from about 0.25 mm (10 mils) on some turbine blades and vanes to over 2.5 mm (100 mils) or more on combustors. The coating can reduce the substrate temperature by 200 or more degrees Fahrenheit (111 degrees Centigrade), depending on the hot and cold side boundary conditions. On blades and vanes, the TBC must protect the airfoil and usually the attachment platform or end walls. On combustors, the TBC is applied on the interior surfaces. The metallic bondcoat can be applied by various methods including thermal spray methods (e.g., shrouded and air-plasma torch, vacuum chamber plasma torch, detonation gun, or high velocity oxy-fuel gun), gas diffusion (such as pack aluminizing), and advanced methods of electroplating. The zirconia ceramic layer can be applied using various methods including thermal spray and electron beam physical vapor deposition (EB-PVD).
In the application of thermal spray coatings on complex shapes, such as turbine blades or vanes, there are several issues that affect the quality of the coating or sometimes even the possibility of applying the coating. Standoff is one such issue because it affects the microstructure of the coating including its porosity and microcracking. Controlled porosity and microcracking are essential to the thermal shock and thermal fatigue resistance of the oxide layer in a TBC. The shape of the part including protuberances (such as the vane platform edges) sets the minimum standoff that can be achieved, which may be too long for the desired microstructure to be achieved using the current state of the art thermal spray devices and shields.
Most TBC metallic bond coats contain one or more very reactive elements such as aluminum or yttrium and, to provide adequate corrosion resistance in service, must be deposited in such a manner that these elements are not oxidized during the deposition process (internal oxidation). As noted, co-axial inert gas shields (e.g., Jackson, U.S. Pat. No. 3,470,347) are a very effective means of accomplishing this. This approach is a much more convenient and a lower cost method of coating reactive metal coatings, such as NiCoCrAlY, than is vacuum or low pressure plasma spray. It is effective, however, for only relative short standoffs, and therefore may not be very effective for some complex parts such as some turbine blades and vanes.
In summary, thermal spray methods are known to those skilled in the art for the deposition of reactive materials such as most metals without significant degradation due to oxidation during deposition. However, these techniques involve either very expensive deposition in vacuum chambers or the use of inert gas shields with limited standoff effectiveness. Moreover, thermal spray methods of deposition of ceramic coatings with desired microstructures are known, but also have limited standoff capabilities. It is also difficult to adequately control the amount and rate of substrate and coating heat absorption and dissipation during coating, particularly when coating large or complex shapes.
It is an object of this invention to provide a novel gas shield or shroud surrounding the effluent of a thermal spray device.
It is a further object of this invention to extend the effective working distance or standoff between the thermal spray device and the surface being coated through the use of said unique gas shield.
Another object of this invention is to provide a method of thermal spraying reactive materials using a unique dual gas shield consisting of an inner inert gas shield and an outer gas shield.
It is a further object of the invention to provide coated articles using the methods of this invention.
It is a further object of this invention to provide a unique gas shield or shroud comprising a combustion flame and combustion products surrounding the effluent of a thermal spray device such as a plasma spray device.